Table of Contents

5.4.3.5 Mid/MidB/Mid$/MidB$ Statement

Note

This section describes the implementation of MS-VBAL §5.4.3.5 Mid/MidB/Mid$/MidB$ Statement.

Syntax

AST node Instruction kind(s) Notes
MidStatementNode Simple Mid, Mid$, MidB and MidB$. See RD-VBAL §3.4.2 Simple Statements.

MidStatementNode records the statement's spelling in two independent flags: IsByteMode distinguishes MidB/MidB$ from Mid/Mid$, and IsStringInput records the $ suffix (Mid$/MidB$).

Spelling IsByteMode IsStringInput
Mid false false
Mid$ false true
MidB true false
MidB$ true true

The replacement-span mechanics of MS-VBAL §5.4.3.5 split only on byte mode (MidB/MidB$ versus Mid/Mid$), never on the $ suffix. IsStringInput is preserved because it mirrors the VBVariant/VBString split of the Mid/Mid$ function overloads, which matters for static semantics.

Static Semantics

The declared type of the target MUST be String or Variant (MS-VBAL §5.4.3.5); another is a TypeMismatch. The target is a variable: a literal, an operator's result, a constant (by its name, or qualified by its project, its module or the library Enum that declares it) or a function of the library (Mid(Left, 1, 1) = "x") is VBC09333 (VariableRequired).

The position and the length are used as the runtime semantics uses them - as Long - and the value as a String, so each has to be Let-coercible to that (RD-VBAL §5.5.1 Let-coercion).

The rule is MidStatementStaticSemantics's, which StatementStaticSemanticsEvaluator asks. An operand whose declared type is not known is deferred, not rejected.

Note

A Byte() target is a run-time possibility the specification's static rule does not allow: it is a TypeMismatch.

Runtime Semantics

Mid replaces a span of the characters of a variable with characters of a value. The replacement never changes the length of the string: the number of characters replaced, x, is the least of the length asked for, the number of characters left in the target from start on, and the number of characters in the value.

  1. The target's value and the value to assign are each Let-coerced to String (RD-VBAL §5.4.3.8 Let Statement). A Variant is what it holds.
  2. start and length are Let-coerced to Long.
  3. If start is less than or equal to 0, or greater than the length of the target, or if length is less than 0, runtime error 5 (Invalid procedure call or argument) is raised. A target with no characters has no start that is in it.
  4. The new string is Let-assigned to the target, so the target can be any variable expression a Let accepts, and a fixed-length String target is given back at its own width.
Spelling Counts in
Mid, Mid$ Characters.
MidB, MidB$ Bytes of the string's in-memory form, as LenB counts them: two to a character, and the trailing byte of a string whose data is an odd number of bytes. A position can fall inside a character, which is split.

The $ suffix changes nothing at run time.

Both modes replace the target's own bytes (RD-VBAL §2.4.1.2 VBStringType), and no text encoding takes part:

  • MidB copies the first bytes of the value over the target's bytes from byte start on. A start may be the trailing byte, which LenB counts. A byte written over half of a character is kept as written, even when the pair it makes is a lone surrogate.
  • Mid copies the first 2x bytes of the value over the target's bytes from byte 2(start - 1) on. A trailing byte of the target follows its last whole character, so it is not one of the x characters replaced, and stays.

With s holding the bytes 41 00 42 (the character A and a trailing byte):

Statement s afterwards
MidB(s, 3, 1) = "Z" 41 00 5A
Mid(s, 1, 1) = "Z" 5A 00 42
Mid(s, 1) = "ZQ" 5A 00 42: s has one character, so one is replaced.
Note

🎯 RD-VBAL differs from MS-VBA in the last row, as the VB6 runtime runs it. That runtime copies bytes up to the target's byte length rather than whole characters, and writes 51, the first byte of Q, over the trailing byte. MS-VBAL §5.4.3.5 replaces characters, and a trailing byte is not one, so RD-VBA leaves it. The other rows agree.

Implementation

Type or member Role
RDCore.Runtime.Semantics.Statements.MidStatementRuntimeSemantics The runtime semantics of all four spellings (RDCore.Runtime).
RDCore.Runtime.Semantics.Statements.StatementRuntimeSemanticsProvider Dispatches a MidStatementNode; see RD-VBAL §3.5.4 Execution.

⏮️ RD-VBAL §5.4.3.4 Erase Statement | ⏭️ RD-VBAL §5.4.3.6 LSet Statement

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